1. Field of the Invention
This invention relates generally to methods and compositions for increasing the brightness retention of a fabric laundered using a hypohalite-generating bleach by employing an amount of bromide-releasing compound sufficient to provide between 0.1 to about 1.5 moles of bromide ion per mole of available chlorine in a wash liquor to mitigate the negative effects of said hypohalite bleach on the optical brightener present in either the wash liquor or on said fabric. The invention also relates generally to fabrics washed using commercial laundry detergents, which typically deliver hypohalite bleach sensitive or unstable optical brighteners. By employing the inventive methods and/or use of the inventive compositions, enhanced protection of the optical brightener is enabled, resulting in increased brightening of the laundered fabrics, measured versus a control as an increase in the Stensby whiteness measure of the fabric by a AW value of greater than about 3.0 versus a control.
2. Description of the Related Art
It is desirable to employ a hypohalite bleach when laundering fabrics in order to bleach stubborn stains, soils and dirt, and to further achieve whitening and brightening of fabrics, particularly of white and light colored textiles and materials, and most particularly of cotton and other natural fiber containing textiles and fabric articles made therefrom. While “whitening” and “brightening” are used somewhat interchangeably to denote overall improved whiteness of a washed fabric, it is generally understood that there are two contributions to the overall improved whiteness effect. Conventionally, whitening may be considered to be the result of removal via detergency and bleaching of colored species, such as stains, pigments, dyes and such from the fabric during a washing and/or bleaching process, while brightening may be considered to be the result of deposition onto the fabric of an optical brightener. Optical brightener deposited onto the fabric results in a perceived enhancement of overall whiteness owing to their inherent properties of generally absorbing ultraviolet wavelengths of incident light (and hence are colorless to the human eye from an absorptive contribution) and emitting longer wavelength light via a fluorescence mechanism. This fluorescence emission, typically at lower energies and hence at visible bluish to reddish wavelengths, effectively contributes a bluish to reddish colored tint to the incident light that is reflected from the fabric surface under illumination, which is then perceived by the human eye and hence the fabric surface is perceptually seen as being a “whiter” white. Since cotton and other fabrics tend to have a slightly yellow or grayish cast, particularly with age and wear of their respective fibers, the use of optical brighteners that deposit onto their surfaces during the wash process can significantly mask this discoloration and provide a pleasing overall white appearance to such treated fabrics.
Use of strong oxidizers such as hypohalites, however, generally result in destruction or oxidation of the optical brightener to a non-functional or tinted derivative that prevents the brightening contribution otherwise provided during laundering, particularly when using commercial laundry detergents that have optical brighteners present that are not stable in the presence of bleach. Attempts to overcome this negative effect generally include the use of more bleach stable optical brighteners, which however suffer from high cost and yet are not fully bleach stable under typical wash conditions. Another approach includes altering the wash conditions to better protect optical brightener in the wash, such as that in U.S. Pat. No. 6,413,925 to Akbarian, et al., which is hereby incorporated by reference, and which teaches the use of an effective level of alkalinity combined with nitrogen bearing surfactants to maintain the wash pH at a value above around 10.5 and preferably near pH 12 under conditions that serve to mitigate the loss of whitener in the presence of a hypohalite bleach.
Use of high wash water pH to overcome negative impacts on brightener and other detersive components however are problematic when lower pH detergents, particularly liquid detergents which contain lower levels of builders and alkalizing agents, are employed in combination with a hypohalite-generating bleach. Some means of preserving and/or enhancing whitening of fabrics in the presence of these oxidizing bleaches that does not require a large modification of the wash liquor pH or alkalinity level would be desirable. Also desirable is a means of achieving improved whitening when using a commercial laundry detergent in combination with a hypohalite-generating bleach. It is also desirable to bleach fabrics with hypohalite-generating bleaches in order to disinfect them without suffering the loss of overall whiteness of the fabrics.